In recent years, there is a growing demand for rubber products that are capable of withstanding harsh usage environments accompanying the higher levels of function and performance in the rubber industry, and particularly automobile parts. Although the properties of rubber products are determined by such factors as selection of the raw material rubber or the combination of compounding agents used, more recently, staple fibers have come to be routinely incorporated for the purpose of improving reinforcability and abrasion resistance.
As shown in FIG. 6, a type of power transmission belt in the form of a V-ribbed belt 10 has core wires 12 embedded in a cushioning rubber layer 14, a covering canvas 15 as necessary above the cushioning rubber layer, and a plurality of ribs 17 and 18 serving as friction transmission portions provided below the cushioning rubber layer 14, and in comparison with other V-belts, demonstrates superior power transmittability corresponding to the increase in contact surface area with a pulley attributable to the presence of the V-shaped ribs, V-ribbed belts are taking the place of V-belts in power transmission applications involving driving of accessories such as automobile air compressors or alternators, and more recently, in addition to the reduced size of engine rooms and reductions in pulley diameter accompanying reduced engine weight, are being applied in accommodation of serpentine drive systems that are driven by suspending belts between a plurality of pulleys arranged so as to wind through the engine room.
In addition, since V-ribbed belts allow the obtaining of high output as a result of having ample bending fatigue resistance due to their shape, they are used at a high initial tension setting since they are used with small diameter pulleys and rotate at high speeds. Recently, the Freon conventionally used in air compressors has been changed to the use of Freon substitutes in consideration of problems associated with environmental destruction. Consequently, it becomes necessary to enhance compressive force in order to cool to the same temperature in comparison with conventional Freon, thereby increasing the torque generated when operating the air compressor and forcing V-ribbed belts to withstand even higher loads.
In the case of this type of belt, power transmission belts have been proposed that enhance lateral pressure resistance of friction transmission portions of the belt by embedding a short fiber assembly in the ribs while maintaining orientation in the direction of belt width, and as a result of further intentionally exposing a portion of the embedded staple fibers from the lateral surface of the belt, friction performance and tackiness of the ribs are controlled with the aim of demonstrating the effect of inhibiting sound generated during belt slippage.
However, in the case of using a rubber composition in which staple fibers have been dispersed in a power transmission belt that is repeated subjected to compressive force, dispersibility of the staple fibers becomes poor resulting in the formation of clumps, and those portions where clumps form become the starting points of cracks, thereby causing belt malfunctions and further shortening the travel service life of the belt.
Moreover, in the case of increasing the number of staple fibers exposed on the belt lateral surface and setting a lower apparent dynamic friction coefficient between the rib rubber and pulley V groove in order to inhibit the generation of abnormal noises during belt slippage, there was the problem of it requiring time until the effect of the staple fibers was eliminated since, following the occurrence of belt slippage, the number of exposed short fibers continues to decrease due to the staple fibers becoming worn, coming out or being cut off. In other words, a longer amount of time until the apparent dynamic friction coefficient between the surfaces of the rubber ribs and pulley V groove increases and becomes stable, and depending on the case, may result in the problem of slippage continuing to the point the belt ruptures due to generation of heat on the surface where slippage occurs.
Patent Documents 1 and 2 indicated below disclose methods for producing a master batch in which staple fibers are dispersed in rubber followed by ultimately incorporating in a rubber composition for the purpose of improving the dispersibility of staple fibers in rubber. However, a softening agent and plasticizer for promoting dispersion are required when producing the master batch, which in addition to increasing the levels of time and labor required to produce the master batch, also result in the problem of the need for additives not directly related to the rubber composition with respect to basic function.
In addition, although Patent Document 3 indicated below discloses an adhesive treatment method consisting of subjecting the staple fibers used to adhesive treatment (such as resorcinol-formaldehyde-latex (RFL) treatment) followed by incorporating in a rubber composition, in this method, the formula of the adhesive used is changed to increase the amount of latex component having high affinity for rubber. However, since the formula of the adhesive is designed with the focus on adhesion, there were limitations on the degree to which dispersibility is improved.
In addition, Patent Document 4 indicated below discloses a method for obtaining staple fibers by forming cotton fibers and the like into the shape of untwisted slivers and then subjecting to RFL treatment and cutting. Although the resulting staple fibers have improved rubber dispersibility, since they are not the result of cutting long fibers, their fiber length and diameter are uneven. In this case, there is the problem of the incorporation of these staple fibers into rubber lowering the effect of being able to control orientation and abrasion resistance of the rubber.
Moreover, Patent Document 5 indicated below discloses a method for obtaining staple fibers by loosely twisting multifilament yarn to form a bamboo reed woven fabric followed by subjecting to adhesive treatment and cutting into staple fibers. However, adhesion uniformity of the adhesive was still not satisfactory and clumps of staple fibers having adhesive adhered thereto became surrounded by staple fibers not having adhesive adhered thereto, thereby preventing this method from solving the problem of the formation of aggregates (lumps). The presence of these lumps inhibited kneading during rubber incorporation resulting in the problem of uneven incorporation of staple fibers. In addition, in the case of these lumps being incorporated in rubber, they form fiber assemblies within the rubber resulting in the problem of creating locations where stress concentrates. Moreover, there was also the problem of staple fibers not having adhesive adhered thereto becoming exposed on the belt surface in the form of staple fibers not completely adhered to the rubber.